1. Field of the Invention
This invention relates to a radiation image storage panel and more particularly, to a radiation image storage panel comprising a support and a phosphor layer provided thereon which comprises a binder and a stimulable phosphor dispersed therein, and optionally a protective film provided on the phosphor layer.
2. Description of Prior Arts
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and a radiographic intensifying screen.
As a method replacing the above-described radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968 has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; exciting the stimulable phosphor with an electromagnetic wave such as visible light and infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the stimulable phosphor as light emission (stimulated emission); photoelectrically converting the emitted light to electric signals; and reproducing a radiation image as a visible image from the electric signals. In the above-described radiation image recording and reproducing method, a radiation image can be obtained with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the case of using the conventional radiography. Accordingly, this radiation image recording and reproducing method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent protective film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (stimulated emission) when exposed to an electromagnetic wave such as visible light or infrared rays after having been exposed to a radiation such as X-rays. In the radiation image recording and reproducing method, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the applied radiation dose, and a radiation image of the object is recorded on the radiation image storage panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by exciting the panel with an electromagnetic wave such as visible light or infrared rays (stimulating rays). The stimulated emissions is then photoelectrically converted to electric signals, so as to produce a visible image from the electric signals.
It is desired for the radiation image storage panel employed in the radiation image recording and reproducing method to have a high sensitivity and to provide an image of high quality (high sharpness, high graininess, etc.), as well as a radiographic intensifying screen employed in the conventional radiography.
The sharpness of the image in the conventional radiography depends on the spread of the emitted light (spontaneous emission) within the radiographic intensifying screen. In contrast to the conventional radiography, the sharpness of the image in the radiation image recording and reproducing method utilizing a stimulable phosphor does not generally depend on the spread of the light (stimulated emission) emitted by the stimulable phosphor within the radiation image storage panel, but on the spread of stimulating rays within the panel. The reason can be described as follows: Since the radiation energy-stored image recorded on the radiation image storage panel is sequentially detected, the stimulated emission given upon excitation with the stimulating rays for a certain period of time is detected as an output from the area of the panel to be excited therewith for said period. When the stimulating rays are spread through scattering or reflection within the panel, the stimulated emission from the area wider than the area to be excited is detected as the output therefrom.
Accordingly, the quality of the image provided by the radiation image storage panel, particularly the sharpness of the image is generally enhanced by making the thickness of phosphor layer smaller, but in this case the sensitivity thereof is apt to decrease. Therefore, for attaining the enhancement of the image quality without decreasing the sensitivity, it is desired that the mixing ratio between the binder and the stimulable phosphor (binder/stimulable phosphor) in the phosphor layer is made smaller so as to give a phosphor layer containing the stimulable phosphor in a large amount.
The radiation image storage panel is also required to have a sufficient mechanical strength so as not to allow easy separation of the phosphor layer from the support (and from the protective film in the case that the protective film is provided on the phosphor layer), when mechanical shocks and mechanical force caused by falling or bending are applied to the panel in the use. Since the radiation image storage panel hardly deteriorates upon exposure to a radiation and an electromagnetic wave ranging from visible light to infrared rays, the panel can be repeatedly employed for a long period of time. Accordingly, it is necessary for the panel in the repeated use not to cause such troubles as the separation between the phosphor layer and support and the separation between the phosphor layer and protective film caused by the mechanical shocks applied in handling of the panel in a procedure of exposure to a radiation, in a procedure of reproducing a radiation image brought about by excitation with an electromagnetic wave after the exposure to the radiation, or in a procedure of erasure of the radiation image information remaining in the panel.
However, the radiation image storage panel has a tendency that the bonding strength between the phosphor layer and the support as well as that between the phosphor layer and protective film decreases as the mixing ratio of the binder of the stimulable phosphor in the phosphor layer decreases, in other words, as the amount of the stimulable phosphor contained therein increases.
For instance, it has been heretofore proposed to employ cellulose derivatives as a binder of phosphor layer of the radiation image storage panel from the viewpoint of dispersibility of the stimulable phosphor in the binder solution (coating solution), but in this case the obtained panel has no mechanical strength enough for preventing easy separation of the phosphor layer from the support. It has been also proposed to employ a polyester resin as the binder of the phosphor layer, but in this case it is difficult to obtain a phosphor layer containing a stimulable phosphor in a large amount.
Further, in the case that a phosphor layer is formed on a support by a conventional coating procedure using the above-mentioned binders, the stimulable phosphor particles are apt to separate from the binder in the drying procedure of the phosphor layer, because the binders have a poor affinity for the stimulable phosphor. As a result, the relatively large amount of phosphor particles gather on the support side of the phoshor layer, and accordingly, the phosphor particles are present in a relatively small amount on the panel surface side of the phosphor layer (or the protective film side, that is, the side which is exposed to stimulating rays and from which the stimulated emission is read out) so as to produce so-called "gathering on surface" of binder. In such radiation image storage panel, especially when the phosphor layer contains the stimulable phosphor in a large amount, the phosphor particles aggregate on the support side of the phosphor layer, whereby the enough bonding strength between the phosphor layer and the support cannot be obtained. In addition, the stimulating rays easily spread on the panel-side surface of the phosphor layer because of the gathering on surface of the binder, so that the quality of the image tends to deteriorate.
On the other hand, in order to enhance the bonding strength between the phosphor layer and protective film in the radiation image storage panel comprising a support, phosphor layer and protective film, it has been proposed to employ the known acrylic resin such as a polyalkyl methacrylate as the binder of the phosphor layer, but there is a tendency that the cracks are produced in the phosphor layer when the mechanical shock such as bending is given to the panel.